Hosted by David L. Jones
And presented in Dave's unique non-scripted overly enthusiastic style!
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EEVblog #102 – DIY Constant Current Dummy Load for Power Supply and Battery Testing
Posted on August 1st, 2010 40 commentsDave grabs a few junkbox parts and builds a useful constant current load for switch mode power supply, battery testing, and other applications.
EEVblog #102 - DIY Constant Current Dummy Load for Power Supply and Battery Testing [ 0:01 ] Download (5819)EEVblog - Official Releases, EEVblog - Podcast analog, design, hardware, microcontroller, power supply, testing40 responses to “EEVblog #102 – DIY Constant Current Dummy Load for Power Supply and Battery Testing”
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Dave another excellent episode. On the audio segment you said you were possibly planning a return to hobbyist kits. I believe this with some of the digital assisted aspects your mentioned would be a wonderful for kit as bench top tools for a hobbyist. While watching I keep wondering if adding in a parallel variable capacitor or some induction would be a good way to show the effects of parasitic effects on power systems.
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I made something simular a while back, only it is a bit bigger :p. It go’s to 16A.
http://www.elektronicastynus.be/Projecten/dummyload/index.php (Dutch)-
some nice projects on your website any chance of translating them all to English
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@Evan
A few off the projects are already translated.
http://www.elektronicastynus.be/english/Projects/index.phpThe rest is on the todo list :p.
Stynus
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Hi Dave and all,
I’m kind of new to electronics and just an example with your circuit, if we wanted to make the heat more uniform instead of it being all on the transistor, would hooking up a heap of diodes (rated 1A) and 2 transistors (rated 1W Tambient) work as the picture below?
http://img72.imageshack.us/img72/3967/diodes.png
Let me know what could go wrong here?
Alex.
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Alex, there are two problems with your circuit:
1. Constant current sources are supposed to keep their output current constant regardless of the output voltage. Series diodes will limit the minimum operating voltage of your circuit.
2. The total amount of power that is dissipated in the circuit is fixed and it does not depend on the components used. All this power is converted into heat and it is the role of the heat sink to push this heat out of the system so that the components stay within their specified operating temperature range. The only thing your circuit achieves is helping the heat sink to stay cooler at the expense of potentially overheating poorly cooled diodes. The correct solution would be to use a larger heat sink, forced air flow or a larger transistor. Look at the Stynus’ website for an example.
I like the way you’re exploring the problem space and asking questions. Both are very important when learning about electronics.
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robert August 2nd, 2010 at 02:12
Just use another MOSFET + heat sink in parallel.
Adding the diodes will force you to up the input voltage to compensate for the _fixed_ voltage drop across them. With that many diodes you couldn’t use it with a 5V power supply.
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I’m a bit surprised you got away without a resistor between the opamp and MOSFET gate to prevent oscillations. The LM324 is only good to drive 50pF and I would have thought the gate capacitance was higher than than. But if it works …
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Chris Jones August 5th, 2010 at 05:29
Hi Dave,
I used to build stepper motor drivers when I was a kid, and one day I made one using this circuit as a constant current sink, but it oscillated. I wonder why yours doesn’t oscillate, maybe it is due to different components or a different external circuit.Eventually I figured out that the reason why mine oscillated was that there is a pole in the opamp due to the internal compensation capacitor, and there is another pole due to the RC circuit formed by the output resistance of the opamp and the gate capacitance of the MOSFET. Each of these two poles gives close to 90 degrees of phase shift at high frequencies, and there are bound to be some other less significant poles due to parasitics which will give a few more degrees so that the total phase shift around the loop due to the poles adds up to 180 degrees at some high frequency, and 180 degrees of phase shift turns what should be negative feedback into positive feedback, and oscillation.
The nicest way of solving this would be to get rid of the pole of the op-amp compensation by using a transconductance amplifier instead, but chip companies for some reason don’t market these much as products (although chip designers use them all the time inside bigger chips, about as often as opamps). The only ones I could find available to buy were CA3080 (obsolete) and LM13700, both of these need dual supplies. It would be possible to build one with a few transistors but the component count would be a bit high then.
It’s good that your one doesn’t oscillate, maybe your FET has less capacitance than the ones I was using, leading to less phase shift. The load can have an effect too, if the drain of the MOSFET is connected to a high impedance load (like the coil of a stepper motor) then the gain from the gate to the drain of the MOSFET is high, and the Miller effect then makes it look like the gate capacitance is even higher.
Chris
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Dave you should make and sell some kits with helpful electronics testing tools just like this one.
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armindilo August 2nd, 2010 at 04:46
webkraller,
If you have a compatible system of paper and marker, I have a free serial key for DaveCAD!
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Karl (not that Karl, the other Karl) August 2nd, 2010 at 05:07
I am actually using an LM317 for a 1A current source / load.
You don’t need a power wire-wound resistor. The same trick you did with the 1 Ohm resistor works, paralleling cheap 1% resistors. I use a bunch of switches to parallel more or fewer resistors to adjust the current.
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Another really cool video, Dave. Quick question, what would be different if you used (or didn’t use) thermal compound between the heatsink and the MOSFET? Would it be very slightly cooler with thermal compound?
Thanks.
~John -
Bearman August 2nd, 2010 at 17:13
I like the DavCAD also. Need to get that ASAFP.
Hey!! I’d like to swap my junk box for yours. All my junk is junk! You’ve got neat junk in yours. How ’bout it!?!?!?
I was just reading up on Op Amps today. Perfect timing for your blogs topic.
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David Robertson August 5th, 2010 at 20:39
(This is me re-posting an earlier comment i made here which was lost (i assume) as Dave transferred the site)
More info on the battery capacity logger would be interesting, Dave.
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Michael Thompson August 6th, 2010 at 19:56
I think I posted one that got lost as well.
How well would this project scale up to handle 10s or 100s of amps?
A buddy is giving me what he calls a “big old industrial power supply” that I’d like to use for my ham radio bench setup, but if it’s that old i think it needs to be looked over very carefully.
This really is a cool project!
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Just found your blog. AWESOME! The kind of circuit walk through is totally invaluable for a hobbyist.
Well done.
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Very nice job. I’ve built and used a dummy load, with just a 10K pot and a 2N3055. Must build one more complete like yours one of these days
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Kostas August 13th, 2010 at 17:38
Hello guys-Dave. Found that blog a couple of weeks ago. Nice work Dave, BTW your accent reminds me of my cousin in Australia.
Guys i’m working on a dummy load myself. I have decided to make it with high power resistors. It will have a maximum power draw of 150 watt. The think that bothers me is if i can use PWM to drive the source current to the load and if yes what will be a good frequency
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Claudio August 21st, 2010 at 19:18
Hi there,
Dave I like your hands-on approach – goes directly to my amteurish brain.
So I just built one of these and it is oscillating (I can see spikes on the +5 rail) and the output from the lm324 is oscillating somewhere between zero V and 1 V. A Filter cap between + and – din’t help:(Any Ideas Anyone?
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Dave C September 9th, 2010 at 15:17
As always, a great blog, Dave.
I followed your heat sink example, but I do not understand why a temperature rise is important.
So we have a wattage being dissipated by the FET and that results in a specific temperature increase, and the heat sink conducts the heat to the air.
Why do we care about the temperature? (I’m sure there’s a reason — please enlighten us.)
Thanks for another great blog!
Dave C.
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Hi Dave,
I’m a hobbyist/DIY’er with primitive electronics knowledge going all the way back to the eighties and never really developed since. Lately I’ve been trying to crash course myself up to speed. I’ve come across your blog and it is quite interesting , so thank you!!!!
Re: the Dummy Load for Power Supply – How would you set up testing a LCD HDTV SMPS seperate from the TV and simulate a load from SMPS that transforms AC source of 120V~240V AC +/- 10% @ 50/60 HZ into DC 5V & 12V& 24V source.
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Hi Dave,
First of all, thanks for excellent blog and excellent videos. I love your latest blog on switchmode DC-DC power supplies. It is very timely and appreciated information for my latest projects.Are you selling any of the blank circuit boards you show in this video?
I have been wanting to build a battery performance logger, and this looks like a pretty nice start.
Thanks,
-slinky -
Doesnt seem to be working on my IE7. Just thrashes incessantly, refreshing the address bar non-stop. View source shows about 6 lines of curious JavaScript. I can be from may laptop but the page its ok on Mozila, i cant undestend someting im on alexa now and your rank is verry big, i found you blog on second page of google .Andrei from Italy
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Hi Dave,
On the similar lines, I think, we can make a programable power supply using a micro-controller. The output from Micro to DAC to Op-amp to transistor. The feedback is measured by ADC to maintain a regulated voltage output. The voltage output can be set by key pad or through commands from serial port!!
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Nice video, I just made one capable of 500 watts, ~40 amps @ 12v for testing computer Power supplies. I needed a ‘slightly’ more complicated cooling solution though…
Check it out:
http://img843.imageshack.us/g/img0733um.jpg/ -
Thanks for sharing your thoughts in that last post. You have a talent for making a hard subject clear to others. I enjoy reading the posts from a guy who has the same flair for explaining things.
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Hi Dave!
Thanks for the idea, just made one myself too. I modified your lm358 based schematic, added current meter on ICL7107, and i must say it works perfect
Range 10mA to 20A, 20A is just a matter of cooling and number of mosfets.Here is the link if you don’t mind, no english description yet http://diy.elektroda.eu/sztuczne-obciazenie-o-regulowanym-pradzie/
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Something to watch out for with this kind of circuit. At the moment the load is being connected, there is no voltage yet over the transistor and the gate of the fet is fully open. This will generate a current peak before the opamp has time to squeeze the gate.
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Craig May 18th, 2011 at 00:50
I’ve watched the video several times and event breadboarded-up the circuit but I’m still unclear about what exactly the MOSFET and the 2nd op-amp are doing.
I understand the the op-amp “wants” it’s inputs to be the same, so if there’s 1V on the “+” input it’s going to do “something” to the output until it sees “1V” on the “-” input.
But what is actually happening here? What is the role of the MOSFET? Why if there’s a 12V load attached to the drain is 12V not seen on the source as well?
Thanks.
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Hey Dave,
Thanks for the idea. I just built up my own variable current load and through it in an Altoids box.
http://embeddederic.blogspot.com/2011/07/everything-including-kitchen-current.html
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Joe Xerox August 13th, 2011 at 07:31
What should or could I do to use this with a PSU arround 50V. The problem I think ‘would arrise is that the Vgs would we out of spec.
Must I go to a P-channel mosfet? But that wouldn’t work with the opamp.
Is there a way arround this?
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Great write-up, I’m normal visitor of one’s blog, maintain up the excellent operate, and It’s going to be a regular visitor for a long time.
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stephen August 1st, 2010 at 18:20